1. Field of the Invention
The present invention relates to an operational amplifier, and more particularly to an operational amplifier having an offset correcting function of correcting an input offset.
2. Description of Related Art
An input offset voltage is one of the features of an operational amplifier. The input offset voltage is, for example, a differential voltage between an input voltage and an output voltage of a negative-feedback operational amplifier. In the ideal operational amplifier, it is ideal that the input offset voltage is zero. For realizing the ideal input offset voltage of the operational amplifier, the input offset voltage is externally adjusted in conventional techniques. The technique of externally adjusting the input offset voltage is disclosed in Japanese Unexamined Patent Publication No. 11-88071 (referred to as “Related Art 1”).
FIG. 5 shows an offset voltage correcting circuit 100 of the Related Art 1. The offset voltage correcting circuit 100 includes a negative-feedback operational amplifier 101 that amplifies an input voltage applied to a noninverting terminal 114 and feeds the amplified voltage to the next stage. The operational amplifier 101 includes PMOS transistors 102 and 103 as a differential pair and NMOS transistors 108 and 109 as a current mirror circuit. NMOS transistors 118 and 119 are connected between source terminals of NMOS transistors 108 and 109 and the ground through offset correcting terminals 115 and 116, respectively. A power supply voltage VDD is applied to gates of the NMOS transistors 118 and 119 through switches 120 and 121, or a control voltage generated with a DAC (Digital Analog Converter) is applied to the gates through switches 122 and 123.
A resistance value (ON resistance value) of the NMOS transistors 118 and 119 in an ON state is changed in accordance with the control voltage. That is, a gate voltage of the NMOS transistors 118 and 119 is controlled based on the control voltage to thereby control a current ratio between the NMOS transistors 108 and 109 that constitute the current mirror circuit. The offset voltage correcting circuit 100 controls the current ratio in the current mirror circuit through the application of the control voltage to control the input offset voltage.
However, the offset voltage correcting circuit 100 of the Related Art 1 needs to use an analog signal voltage as the control voltage. Hence, in the case of controlling the input offset voltage with a controller such as a microcomputer, a digital control signal output from the microcomputer should be converted into an analog control signal by use of a converter such as a DAC. Further, if the microcomputer includes no DAC, a DAC should be added between the operational amplifier and the microcomputer.
In general, the DAC occupies a very large area. Even if the DAC is externally connected, it is necessary to set aside a given substrate area for the DAC. As a result, it is difficult to downsize a device or reduce a cost in a system that requires the DAC.